Subterranean drilling systems that employ downhole drilling motors are commonly used for drilling boreholes in the earth for oil and gas exploration. FIG. 1 is a schematic isometric cutaway view of a prior art subterranean drilling system 100. The subterranean drilling system 100 includes a housing 102 enclosing a downhole drilling motor 104 that is operably connected to an output shaft 106. A thrust-bearing apparatus 108 is also operably coupled to the downhole drilling motor 104. A rotary drill bit 112 configured to engage a subterranean formation and drill a borehole is connected to the output shaft 106. The rotary drill bit 112 is shown as a roller-cone bit including a plurality of roller cones 114. As the borehole is drilled, pipe sections may be connected to the subterranean drilling system 100 to form a drill string capable of progressively drilling the borehole to a greater depth within the earth.
The thrust-bearing apparatus 108 includes a stator 116 that does not rotate and a rotor 118 that is attached to the output shaft 106 and rotates with the output shaft 106. The stator 116 and rotor 118 each include a plurality of bearing elements 120 that may be fabricated from polycrystalline diamond compacts (“PDCs”) that provide diamond bearing surfaces that bear against each other during use.
In operation, high-pressure drilling fluid is circulated through the drill string and power section (not shown) of the downhole drilling motor 104, usually prior to the rotary drill bit 112 engaging the bottom of the borehole, to generate torque and rotate the output shaft 106 and the rotary drill bit 112 attached to the output shaft 106. When the rotary drill bit 112 engages the bottom of the borehole, a thrust load is generated, which is commonly referred to as “on-bottom thrust” that tends to compress the thrust-bearing apparatus 108. The on-bottom thrust is carried, at least in part, by the thrust-bearing apparatus 108. Fluid flow through the power section may cause what is commonly referred to as “off-bottom thrust,” which is carried, at least in part, by another thrust-bearing apparatus that is not shown in FIG. 1. The drilling fluid used to generate the torque for rotating the rotary drill bit 112 exits openings formed in the rotary drill bit 112 and returns to the surface, carrying cuttings of the subterranean formation through an annular space between the drilled borehole and the subterranean drilling system 100. Typically, a portion of the drilling fluid is diverted by the downhole drilling motor 104 to cool and lubricate the bearing elements 120 of the thrust-bearing apparatus 108.
The off-bottom and on-bottom thrust carried by the thrust-bearing apparatuses can be extremely large. The operational lifetime of the thrust-bearing apparatuses often determines the useful life of the subterranean drilling system 100. Therefore, manufacturers and users of subterranean drilling systems continue to seek improved bearing apparatuses.